Renewable energies became more and more important in the last years. The production of biogas using agricultural waste and the use of wind and solar energy in combination with water electrolysis is one way to substitute natural gas. Therefore the number of syngas plants is growing very fast. On the other hand, the operation of such plants could be responsible for a significant number of accidents. The main focuses of this contribution are the explosion characteristics and hazards arising from the biogas. Primarily, these are the hazards of fire and explosion induced by flammable components of syngas. However, further hazards are the dangers of asphyxiation and poisoning by gases such as carbon monooxide. These hazards will be the aim of the following article. In order to prevent explosions when storing and handling syngas it is necessary to know the explosion limits of individual gas components and its gas mixtures in mixture with air. However, syngas from gasification unit can vary significantly in its composition. Therefore, for each gas composition the explosion limits would have to be determined. This would require a considerable amount of time and effort. Due to this fact, the explosion limits of syngas are frequently referred to only by the hydrogen fraction of the gas mixture in the safety-relevant literature. In reality as syngas consists of hydrogen, methane, carbon monoxide, carbon dioxide and further residual gases the explosion limits are generally over or underestimated.

1. Introduction

Biomass gasification is the thermal conversion of a heterogeneous solid material into a gaseous intermediate fuel, syngas, consisting primarily of carbon monoxide and hydrogen that can be used for the production of heat, power, liquid fuels, and chemicals. Biomass is the only renewable source of organic carbon, and its use is a key issue of sustainable development. The gasification of biomass has the potential to offer a major contribution to meeting the international targets for CO2 mitigation (Knoef et al., 2012).

Gasification is also called “staged combustion”, since usually the syngas is produced with the intent to burn it later. This raises the question why first gasifying and then burning the gas over direct combustion of the biomass. There are several essential advantages: possibility to transport in pipelines; ease of control and continuous operation; clean combustion of produced gas since impurities can be removed from the producer gas, and the volume of producer gas is much smaller compared to flue gas; efficient and clean combustion since the exact required air can be mixed for optimum combustion; producer gas can be used in engines or turbines with higher efficiencies over steam devices; producer gas, in particularly syngas, can be used for chemical synthesis like fertilizers and transportation fuels (Reed and Guar, 2000).

The characteristic safety parameter of syngas from biomass gasification in a closed vessel explosion, so called explosion characteristic, discussed in this contribution is the maximum explosion pressure. The maximum explosion pressure is the highest explosion pressure over the flammable range in a closed volume at a given fuel concentration (Eckhoff, 2005).

These explosion characteristics are important for design of safety devices (e.g. relief systems, vents), able to ensure active protection of pressure vessels where flammable mixtures are formed. Beyond safety devices, the values of these parameters are useful for emergency planning especially for developing scenarios where emergency relief or external heat transfer may be inadequate. At the same time, the maximum explosion pressure that the explosion reaction can generate is one good measure of the magnitude of the hazard associated with the reaction (CCPS, 1995).

The present contribution presents absolute explosion pressures (in terms of bar(a)) of stoichiometric H2/CH4/CO/C3H8/CO2/O2/N2 syngas mixtures with air calculated for various initial temperatures and pressures. The aim of this contribution is to evaluate the influence of the temperature on the explosion parameters, namely maximum explosion pressure, of the syngas produced by generator during autothermal gasification in fixed-bed. The primary outcomes are: explosion parameters at ambient conditions and elevated temperatures and pressures.

2. Basic description of technology

The autothermal gasification technology at the Energy Research Center was developed as a result of the Project “Biomass Gasification for Cogeneration“. The aim of the Project was using the generated gas in a cogeneration unit with a piston combustion engine for the electric energy and heat production. Base of this technology is a fixed-bed gasification generator. The fuel is supplied to the grate of the gasification reactor from a fuel bin by the two screw conveyors. The fuel is supplied to the reactor´s lower part automatically based on a measured temperature in the gasification reactor. The generated gas is transported to the hot filter, where are removed dust particles. Behind the filter are gas cooler, exhaust fan and combustion chamber, in which the producer gas is combusted.

The technology is operated in the low under pressure and it is provided by a necessary measurement and control technological units. Thermal output of the autothermal gasifier is 100 kW. As a fuel for testing was used wood pellets with diameter 6 mm. Gas for analysis was sampled from sampling point 2 depicted in Figure 2.

Figure 2: Technological scheme of autothermal gasification unit

The mixture composition investigated in this contribution is given in Table 1.

Ingredients

H2

CH4

CO

C3H8

CO2

O2

N2

Content vol. %

15.85

2.10

20.15

0.99

11.83

0.12

48.96

Table 1: The compositions of syngas for analysis

Due to the complexity of the involved physical phenomena and to the lack of an adequate amount of reliable experimental data, a very limited number of different models and calculation procedures for estimating the physical consequences following the explosion of a gaseous state mixture as syngas are presently reported in the literature. The following model was used to investigate and quantify the role of initial temperature and pressure in affecting the explosion behavior of syngas mixture.

3. Previous studies and analysis

Mathematical model used in this study computes adiabatic flame temperatures and constant volume adiabatic explosion pressures at various initial temperatures and pressures, taking into account 26 species. It has been shown (Skrinsky et al., 2015) that the model is able to predict, with a reasonable accuracy in different fuel-enriched conditions, for different types of gaseous mixtures explosions.

Element balanced equation:

(1)

where ajare moles of j-th element (atom); niare moles of i-th species; Ai,jnumber of j-th element in i-th species; l is the number of elements; j is the number of species.

The free energy (Helmholtz function) of the system F equation:

(2)

where F is the free energy (Helmholtz function) of the system; ni are moles of i-th species; fi is free energy of i-th species.

(3)

where fi is free energy of i-th species; λi are the Lagrangian multipliers for each element (j=1 to J); Ai,jnumber of j-th element in i-th species.

(4)

where gi is the Gibbs free energy for single i-th species; ΔHfis the enthalpy of formation of the i-th species at standard conditions;

(5)

where giis the Gibbs free energy for single i-th species; R is the universal gas constant; T is temperature; ni is the number of moles of every species; V is the volume.

(6)

where ajare moles of j-th element (atom); niare moles of i-th species; Ai,jnumber of j-th element in i-th species; l is the number of elements; j is the number of species; gi is the Gibbs free energy for single i-th species; λi are the Lagrangian multipliers for each element (j=1 to J); R is the universal gas constant; T is temperature; ni is the number of moles of every species; V is the volume.

(7)

where gi is the Gibbs free energy for single i-th species; Ai,jnumber of j-th element in i-th species; l is the number of elements; λi are the Lagrangian multipliers for each element (j=1 to J); R is the universal gas constant; T is temperature; ni is the number of moles of every species; V is the volume.

The equations 1-7 were used to investigate and quantify the role of initial temperature and pressure in affecting the explosion behaviour of syngas-air mixtures.

4. Results and discussions

Results of explosion experiments depend on many different parameters of the investigated process, such as the energy and type of ignition source, size and shape of explosion chamber, initial temperature, initial pressure and composition of the flammable mixture. To ensure the compatibility of data we selected the results for experiments that are in agreement with EN 13673-1. The results of experiments are summarized in Table 2.

A) Experimental and theoretical Pmax for H2/CH4/CO/CO2/O2/N2 at ambient temperature and pressure

From the numerical results of Table 3 it is possible to identify that the increase in the initial temperature lowers the maximum explosion pressure, and increases the flammability range. The value of the explosion pressure with varying H2/CH4/CO/C3H8/CO2/O2/N2 concentration is similar at all investigated initial temperatures. The maximum value of the explosion pressure is found close to 48 vol. % of syngas for all conditions. Further, in Table 4, we reported simulations on the explosion properties of H2/CH4/CO/C3H8/CO2/O2/N2 mixtures at various initial pressure and ambient initial temperature.

From the data performed in Table 4, it is clear that the increase of initial pressure has significant effect on explosion pressure when only the air served as oxidant. This is a good approximation at initial pressures i.e. up to 15 bar(a), but we may assume that will be increasing wrong at higher pressures.

C) Teoretical effects of syngas explosion at ambient and elevated pressures and temperatures

The destructions on structures such as described in Table 5, showing pressure levels and corresponding damages, have been established from tests or actual accidental explosions. The data in Table 5 provide only a small sample on information from various sources. A closer study shows that the greatest part leads us back to information taken out of reference (GreenBook, 1997) in which primarily the consequences of nuclear explosions have been investigated. The damages shown in Table 5 correspond to the pressures given in this table. From these data it is possible to obtain a rough idea about the damages which could be expected. The quantity used for the explosion damage decription is explosion pressure.

Description of damage

Explosion pressure
bar(a)×10-2

Connections between steel or aluminium plates have failed

7-14

Minor damage to steel frames

8-10

Walls made of concrete blocks have collapsed

15-20

Collapsad of steel frames and displacement of foundation

20

Industrial steel self-framing structure collapsed

20-30

Cladding of light industry building ripped-off

30

Brickstones walls, 20-30 cm, have collapsed

50

Displacement of a cylindrical storage tank, failure of pipes

50-100

Table 5: The consequences of explosion effects on structures from empirical data

5. Conclusion

The adiabatic explosion pressures H2/CH4/CO/C3H8/CO2/O2/N2 mixture with air at various initial temperatures and pressures were calculated. The model predictions for the syngas mixtures are compared for four different initial temperatures. Although the results from the evaluation indicate that presented theoretical simulations can become a valuable tool for rough estimation, the modeling requires further improvements to be useful for consequence modeling and design of industrial facilities. Thus, at the first stage, the equilibrium calculations can be used as a rough calculation of a worst case scenario. At the same time, these values will be used as approximate initial values for explosion experiments carried out in heated 1 m3 explosion apparatus designed by OZM Research s.r.o. at Energy Research Centre, VŠB - Technical University of Ostrava. As the practical outcome these results will apply for the syngas produced by generator during autothermal gasification in fixed-bed. The results represents a continuation of numerous efforts by various research groups, where the key underlying problem has been the understanding of results obtained in laboratory tests for predicting the consequences of multicomponent gas mixture explosion scenarios in industry (Skřínský et al., 2014; Skřínský et al., 2015; Skřínská 1 et al., 2014; Skřínská2 et al., 2014).

6. Acknowledgement

This work would not have been possible without the financial support of two grants: (1) Innovation for Efficiency and Environment, reg. no. ED0036/01/01 supported by Operation Program Research and development for Innovation and financed by the Ministry of Education, Youth and Sports; (2) Innovation for Efficiency and Environment - Growth, reg. no. LO1403 supported by National Programme for Sustainability and financed by the Ministry of Education, Youth and Sports. J.V. is thankful for the financial support of the project “Support research and development in the Moravian-Silesian Region 2014 DT 1-International research teams” 02722/2014/RRC, financed from the budget of the Moravian-Silesian Region. J.S. is thankful to his advisors Ing. Jan Koloničný, Ph.D. and doc. Dr. Ing. Břetislav Janovský, CSc. for their knowledge support in the area of decision process, industrial explosion experiments and enthusiasm for a team work.

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